Allostery: Haemoglobin (part 3)

Allosteric mechanism

(It will be useful to refer to the RasMol rendition of the oxyhaemoglobin
structure, with labelled helices, while reading what follows. This rendition
is provided in Part 2)

A simple reduction in the radius of the Fe ion is not sufficient to allow it to
fit into the haem ring. Movement of the ion into the plane is prevented by
steric repulsion between the proximal histidine side chain and the haem
nitrogen atoms. This can only be avoided by a change in orientation of the
histidine, which requires movement of the F helix to which it belongs. In fact
the conformations of the turn between helices E and F, and the FG turn, also
alter besides the shift in the position of helix F. The Tyrosine at the end of
helix H (Tyr 140 in alpha subunit, Tyr 145 in beta) is expelled
from the pocket between helices F and H, which leads to the breaking of the
salt links between the chains.

The important interfacial structural changes are those between the
two heterodimers (i.e. between the alpha 1 - beta 1 dimer, and the alpha 2 -
beta 2 dimer), rather than between the two subunits within each of these dimers.
Note that three of the four salt bridges listed in the section on the
T state occur between the two heterodimers.
Different hydrogens bonds stabilize the alpha 1 - beta 2 interface in the two
different states (Exercise: identify these different hydrogen bonds)
. There is a 15° rotation of the two
heterodimers relative to each other, as indicated below.

Therefore, binding of an oxygen molecule to the Fe ion in the haem ring leads
to marked structural changes elsewhere in the molecule.
The R-state tertiary structure of an oxygenated subunit stabilizes the
same tertiary structure in the other chains, which therefore have a higher
affinity for oxygen.

Because the key to cooperativity is the interaction between heterodimers,
mutations in the interfacial residues between subunits alpha 1 and beta 1,
and between alpha 2 and beta 2, do not tend to affect the cooperative binding
system in the same way that mutations in the alpha 1 - beta 2 interface do.

The significance of the contacts between heterodimers also
explains the observed increase in oxygen affinity. Suppose that a single
subunit, say alpha 1, binds an oxygen and alters from the T to the
R state. Bearing in mind the list of broken
salt bridges listed previously, the interactions between
(a) alpha 1 and alpha 2, and (b) alpha 1 and beta 2, will change. This would
result in the R state tertiary structure in subunits alpha 2 and beta 2,
so that the affinity of both for oxygen is increased. Indeed, the affinity for
the third oxygen molecule is not markedly higher than that for the second, but
both are much greater than that for the first. With oxygens now bound to
alpha 1, alpha 2 and beta 2, the R state tertiary structure will also
be stabilized in beta 1 (due to the tertiary structure of alpha 2 breaking
the salt bridges between it and beta 1; refer to the
previous list). The final empty chain will thus have a very stable R
tertiary structure; this results in the dramatically higher affinity for the
fourth oxygen.